Paper No. 13
Presentation Time: 8:00 AM-12:00 PM
GEOCHEMICAL AND PETROLOGIC CONSTRAINTS ON PALEOPROTEROZOIC BATHOLITHS OF THE PENOKEAN OROGEN, MN & ONT
The Penokean orogen is exposed from Minnesota to northern Ontario and is a result of long-lived convergence along the southern margin of Laurentia during the Paleoproterozoic. Recent geochronologic investigations of the timing of tectonothermal events document protracted plutonism from 1800-1750 Ma associated with Yavapai subduction flip, with a general age trend that becomes younger to the south and east. Slab retreat during ongoing subduction, resulting in the trenchward migration of the locus of magmatism, likely caused the geographical age trend. Notably, three geon 17 batholiths intrude the orogen: The east-central Minnesota batholith (ECMB; 1775 Ma), and the Cutler and Killarney batholiths (1750 Ma) in northern Ontario. Twelve granitoid samples were collected from these batholiths, as well as the 1750 Ma Eden Lake Complex (N. Ontario), in order to perform petrologic and major-, trace-, and rare-earth element analyses. Granitoids intrude into upper greenschist facies supracrustal units, likely derived from a mixed source of supracrustals and subjacent accreted arc material. Rock types include granodiorites, granites, and alkali granites, with major-element classification ranging from calc-alkaline to shoshonitic, all characteristic hallmarks of a subduction-related genesis. Samples from the ECMB (Hillman, St. Cloud, Rockville, Richmond, and Reformatory phases) as well as the Eden Lake and Killarney suites were determined to be volcanic arc granites (VAG) on tectonic discrimination diagrams; two phases of the Cutler batholith and the Watab phase of the ECMB were equivocally concluded to be syn-collisional granites (COLG) and are more evolved with respect to the REE than those classified as VAG. These results illustrate that trace element tectonic discrimination correlates the intrusive bodies to the local terrane lithology from which they were derived and into which they were emplaced. Results and are consistent with other contemporary intrusives from the central part of the orogen: VAG rocks intrude the less evolved island arc / back arc of the orogen, and COLG rocks primarily intrude deformed and metamorphosed Proterozoic metasedimentary sequences farther inboard of the Yavapai margin.